A monogenetic volcano is the product of a single eruptive episode lasting a few hours, or a few years. Basaltic scoria cones are good examples of monogenetic volcanoes and are found in thousands all around the world. They all have the same distinctive morphology. They are rarely more than two or three hundred meters high, and are often asymmetrical; either elongated along a fissure, or else higher on the side that was downwind at the time of eruption. A distinctive feature is their simple geometric profile, defined by the angle of rest for loose scoria: all young scoria cones have side slopes close to 33 degrees. Their craters are large in relation to the size of the edifice as a whole (compare Agua in Slide 5--Part I). Naturally, their crisp profiles soften with age, but the ratio of crater width to basal width changes remarkably little. Thus, scoria cones remain easily recognizable, even after millennia of weathering. La Poruña in the Atacama desert of Chile is 300 m high and appears pristine, but because of the hyperarid environment it may be several thousand years old. A train on the Antofagasta-La Paz railroad (in the shadow near the cone) provides scale. (Figs. 7.8; 16.1).

Part 1       Part 2       Part 3       Part 4       Next Slide


 To VolcanoWorld